Yes, on OS X - the technology used to make most programs is Objective C where messages get passed from the UI to the logic and a model-view-controller idiom is used.

If you are skilled, you can do things like patch the binary to swizzle a method or use SIMBL to modify apps that do not have a plug in system like Keynote.

These reverse-engineering feats require some very detailed skill and can break each time the app is updated, so something that is theoretically possible becomes unlikely to happen as something you can download for free with the work already being done for you.

Your commend about the quicklook ability to preview keynotes is where I would start if you wanted to develop a true application to read keynote files. From there, you would need to reverse engineer the storage format by discovering that a .key file is really just a fancy zip file and go from there.

Thank you for your response. Building a reverse engineered Keynote viewer from scratch is not feasible, in any realistic time-frame. I am not familiar with SIMBL, and I will look into this. Thank you for the informative input.
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user36857Dec 22 '12 at 21:12

Most people are fine with unzipping the file and using the previews. You can also print a keynote to PDF (or render a quicktime movie of the presentation) if you need higher resolution slides to sling around. Both have clear downsides that are obvious since you are asking for an actual file viewer :)
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bmike♦Dec 22 '12 at 21:19

Using a PDF is not an option, as e.g. transitions are lost. It seems as though Quick Look or SIMBL is the way to go. Thank you for your input.
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user36857Dec 22 '12 at 21:24

As you said, Quick Look is the way to go. But the support for eg. advanced animations isn't perfect I think. You could also try to export your .key to HTML if you have direct access, and via injected js take the control over the playback (The html is actually images that are animated through js/css). Put it into a NSWebView or something.

Thank you for your response, and observation regarding Quick Look. Exporting to HTML is not an option as the presentation loses fidelity, compared to the Keynote version.
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user36857Dec 22 '12 at 21:06